The present invention relates to a rotary valve used as a direct drive type rotary servo valve and for a hydraulic pressure control unit of a rolling machine and a method of making such rotary valve.
A conventional rotary valve of the aforementioned is described in, for example, Japenese Patent Unexamined Publication No. 61-153073, wherein axially extending grooves are formed in an outer surface of a cylindrical valve body and in an inner surface of a casing for receiving the valve body, with a valve portion for forming control orifices with edges of side surfaces of the inner and outer grooves, and with a disc-shaped rotator integrally coupled to the end portion of the valve body to drive the valve body and to change an opening area of the control orifices.
Also, as described in that publication, sleeves are provided in parallel with a rotary center axis of the valve body, and flow paths are spaced from each other by the sleeves and are formed on both sides of the valve body. On the other hand, cylindrical holes having an inner diameter substantially equal to an outer diameter of the sleeves are formed coaxially with the sleeves in the casing. A disc-shaped rotator is integrally coupled to a part of the valve body to drive the valve body. The relative motion between the sleeves and the cylindrical holes causes a charge in the opening area of the control orifices, formed by the outer edges of the sleeve, the inner edges of the cylindrical holes and the inner and outer edges of the flow paths.
In the above-described prior art, it would be difficult or impossible to ensure a high manufacture precision since the inner diameter portions of the casings require an intricate manufacture. This makes it impossible to ensure a high precision in control. Also, it would be difficult to reuse the valve portion in the case where edges of the control orifices are worn due to the use for a long period.
On the other hand, if the flow paths are formed in the valve body, there is a problem that the size of the valve body would be enlarged in order to increase the cross-sectional area of the flow paths to ensure satisfactory flow rate control characteristics. Also, since the sleeves are provided in the valve body, it is necessary to provide partition walls and outer walls having mechanical strength enough to suppress any deformation of the valve body in high pressure parts. In addition, since the structure of the valve body is intricate, a stress concentration would be likely to be generated particularly in connection parts between the partition walls and the outer walls. In order to avoid this defect, it is necessary to provide additional reinforcements. As a result, the valve body would be enlarged in size to increase inertia moments, and it would be impossible to ensure a high response time.
Also, as described above, if the inertia moments of the valve body would be increased, the drive, force is required to obtain a desirable response time would increase. It is, therefore, necessary to enlarge the rotator to increase the output of the drive portion and as a result, since the inertia moments of the rotator per se would be increased, it is further difficult to ensure a higher response time.
In general since the sleeves for defining the control orifices in cooperation with a spool and receiving the spool require an intricate and precise manufacture, it is difficult to manufacture the sleeves. In particular, there is a problem that a high technique and a number of manufacturing steps are needed to ensure the positional relationship of the edges for determining the lap amount. Also, after a long service, if the edges of the control orifices would be worn, it is difficult to again use the worn valve portion with additional machining for reuse.